A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) of a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Lithographic apparatus may be of the transmissive type, where radiation is passed through a mask to generate the pattern, or of the reflective type, where radiation is reflected from the mask to generate the pattern. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the scanning-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In general, there is a non-uniformity in the intensity of radiation which is imaged onto the substrate in such apparatus. This is typically caused by, for example, the mirrors or lenses of the projection system having differing reflectivity or transmission over their surfaces. In the case of conventional lithography, so-called deep-UV (DUV), a transmissive filter is included which corrects for this non-uniformity. In the past the properties of the filter were fixed and could not be changed over time. In newer systems the filter is adjustable, and can be adjusted to take account of slow variations in beam uniformity, for example caused by gradual degradation of lens surfaces.
A known adjustable uniformity correction unit for DUV comprises two transmissive plates which are considerably bigger than the projection beam. Different transmission profiles are provided on the plates, so that, when the transmission of the plate is to be adjusted, the point at which the projection beam intercepts the plate is changed by moving the plate. The plates are made from glass and are heavy, consequently their movement is slow. In any event, they are designed and intended to be used to correct for very slow variations.
In extreme ultraviolet (EUV) lithography, there are no materials available which can be used in a transmissive way. Accordingly an arrangement is disclosed in U.S. Pat. No. 6,741,329 in which non-transmissive blades, commonly called venetian blinds (‘blades’), are used to adjust the beam to correct for non-uniformity in the intensity of radiation imaged onto the substrate. In the simplest case the blades are in the form of a series of rectangles that are rotatably mounted and are spread across the projection beam. In more complicated cases the blades can have a more complicated (‘asymmetric blades’) shape. In order to reduce the beam intensity in a given location, the blade at that location is rotated so that it partially blocks the beam. The blades are typically located a distance D≧B/tan(asin(NA)) mm below the reticle where B is the distance between the blades and NA is the numerical aperture at reticle level. If the blades were to be located closer to the reticle, then sharp images of the blade edges would appear on the substrate. Conversely, if the blades were to be moved further away from the reticle, then the spatial frequency of the intensity correction provided by the blades would be reduced.
The blade arrangement of U.S. Pat. No. 6,741,329 does not allow the uniformity or the intensity of the radiation incident on the substrate to be varied in the direction in which the substrate is scanned by the projection beam during a scan. Instead the energy per laser pulse is varied during the scan to generate a varying intensity profile in the scanning direction. However, unlike DUV lithography sources, EUV lithography sources are not capable of changing their output power, and there is therefore no simple way in which the overall intensity of the projection beam incident on the substrate can be varied.
The illumination slit that is used to expose the substrate during scanning is usually curved, as shown in FIG. 2. The blades are oriented with a fixed angle of typically around 60 degrees with respect to the non-scanning direction or x-axis. Due to this angle the shadow of each blade is beneficially spread out in the non-scanning x-direction. Since the overlap of the blades with the slit is different on the left hand side to the right hand side of the slit the transmittance of the blades is not the same. The blade on the left hand side of FIG. 2 is more nearly perpendicular to the slit, leading to a narrower spatial profile with a relatively low transmission, whereas the blade on the right hand side of FIG. 2 is more nearly parallel to the slit, leading to a broader spatial profile with a relatively high transmission. By rotating the blades progressively less from left to right with respect to the x-axis, their peak transmission can be made the same. For example, at the left hand end of the slit the blades may be mounted at 60° relative to the x-axis, whereas at the right hand side of the slit the blades may be mounted at 45° relative to the x-axis.
In order to detect the positions of the blades for the purpose of controlling the amount of attenuation applied by the blades, each blade may have an associated position detector to detect a quantity of radiation received from a radiation source providing a radiation beam that is arranged to be interrupted by a portion of the blade, or an element connected to the blade so as to rotate with the blade in such a manner that the quantity of radiation reaching the position detector is indicative of the orientation of the blade. The outputs of the position detectors can then be supplied to an electronic controller to control the actuators, used to tilt the blades, in such a manner as to accurately orient the blades according to the degree of attenuation desired. Generally the number of radiation sources used in such a position detection arrangement will correspond to the number of blades whose positions are to be detected. Thus, if in a typical arrangement 30 blades are provided, the position detection arrangement may include, for example, 30 radiation-emitting diodes to emit radiation and 30 photodiodes to detect the radiation after attenuation by the blades. All the components may be disposed in a vacuum so that, because of the lack of convection, cooling can present a problem.
When a high measurement accuracy is required, the use of multiple radiation sources can be disadvantageous in that the intensity of radiation emitted can vary from source to source and with time depending on the different thermal behavior of each source, which may cause the relationship between the proportion of radiation received by each detector and the precise orientation of the blade, as well as the angular distribution of the radiation and the degree of self-heating, to vary from source to source. The thermal drift of the radiation emitting diodes can also render these unsuitable for use in high measurement accuracy system. The use of multiple radiation sources is also disadvantageous in so far as it requires use of a high level of components and cabling, as well as providing high power consumption and cooling requirements.